It's been a while since I looked at the Clojure source, but from what I recall, the initialization problem is almost wholly related to loading all of clojure.core, not garbage collection.

If you're hitting slowdowns after initialization, there's a good chance it's reflection, not garbage collection. Type hinting can help if that is the case.

Because of the way that Clojure reuses immutable data structures, it doesn't do as much garbage collection as one might think.